1. Field of the Invention
The present invention relates to a laser oscillator and particularly to a mounting structure for the optical element thereof.
2. Description of the Related Art
FIG. 14 is a structural cross-sectional view illustrating a laser oscillator, utilizing a conventional unstable resonator, disclosed in Japanese Patent Laid-Open No. 1991-257979. In FIG. 14, reference numerals 51 and 52 denote a totally reflecting mirror (curvature radius R1=1 m) and a magnifying mirror (curvature radius R2=2 m), respectively, each having a concave contour and being formed, for example, of Cu; the totally reflecting mirror 51 and the magnifying mirror 52 are arranged spaced apart from each other by a resonator length of 1.5 m in such a way as to be configured in a negative-branch and confocal manner (magnification rate=2). Reference numerals 53 and 54 denote respective mirror pressers on which the totally reflecting mirror 51 and the magnifying mirror 52 are mounted; reference numerals 55 and 56 denote respective mirror cooling plates that are formed of a flat plate and are in contact with the totally reflecting mirror 51 and the magnifying mirror 52 so as to cool them.
Reference numeral 57 denotes a scraper mirror formed, for example, of Cu; through the scraper mirror 57, a ring-shaped laser-beam output is obtained from the unstable resonator. Reference numeral 58 is a transmission mirror formed, for example, of ZnSe; a laser bean passes through the transmission mirror 58 and is emitted outward from the laser oscillator. Reference numeral 59 is a laser medium; for example, in the case of a gas laser such as a CO2 laser, a gas medium excited through discharge or the like is utilized; in the case of a solid-state laser such as a YAG laser, a solid-state medium excited by a flash lamp or the like is utilized. Reference numeral 60 denotes an aperture for controlling the diameter of a laser beam; reference numeral 61 denotes a case for covering the unstable resonator; reference numeral 62 denotes a laser beam generated inside the unstable resonator configured with the totally reflecting mirror 51 and the magnifying mirror 52; reference numeral 63 denotes a ring-shaped laser beam emitted outward through the transmission mirror 58.
FIG. 15 is a cross-sectional view illustrating a mounting structure for the magnifying mirror 52 illustrated in FIG. 14. In FIG. 15, reference numerals 52a and 52b denote an opening diameter and a mirror diameter, respectively, of the magnifying mirror 52; the portions (52b-52a) correspond to thermal-contact portions. Reference numeral 64 denotes a bolt for fixing to the mirror cooling plate 56 the mirror presser 54 on which the magnifying mirror 52 is mounted. Reference numeral 65 denotes an O-shaped ring provided behind the magnifying mirror 52; by pressing the magnifying mirror 52 through the O-shaped ring 65, the magnifying mirror 52 is pressure-bonded to the mirror cooling plate 56, so that the thermal contact between the magnifying mirror 52 and the mirror cooling plate 56 is ensured. In addition, in FIG. 15, the mounting structure for the magnifying mirror 52 (optical element) is illustrated; however, the mounting structure for the totally reflecting mirror 51 (optical element) is the same as the foregoing structure.
Next, the operation will be explained. The laser beam 62 reciprocates between the totally reflecting mirror 51 and the magnifying mirror 52 and is amplified by the laser medium 59 while reciprocating. Part of the laser beam 62 that has been amplified in this manner is reflected by the scraper mirror 57 and emitted outward from the laser oscillator, through the transmission mirror 58. Because the unstable resonator is configured through a confocal arrangement, the emitted laser beam 63 becomes a parallel beam. In this case, the totally reflecting mirror 51 and the magnifying mirror 52 are slightly capable of absorbing laser light; therefore, while the laser beam 62 reciprocates within the unstable resonator, the heat is absorbed by the totally reflecting mirror 51 and the magnifying mirror 52. The heat is radiated from the respective contact surfaces in the surfaces of the mirror cooling plates 55 and 56 that are cooled with water, thereby preventing the mirror temperature from rising.
FIG. 16 is a principal-part cross-sectional view illustrating the mounting structure, for a mirror (optical element), disclosed in Japanese Patent Laid-Open No. 1996-257782. A mirror 82 is mounted in such a way that the reflection surface thereof abuts on a bend block 83. It is required that the foregoing mirror that is utilized in the laser oscillator should have a high flatness of not more than one-tenth of the wavelength of laser light to be oscillated. Ultrahigh-precision flat-surface machining intended for a flatness of not more than 1 μm is applied to a contact surface (flat portion for mounting the mirror) 85 between the bend block 83 and the mirror 82 so that a profile deformation of the mirror 82 due to pressing force is prevented.
In addition, reference numerals 81, 84, 86, 87 and 88, 89, and 90 denote laser light, a supporting base, a mounting screw, adjusting screws, a cooling water path, and a dust-proof member. Additionally, also in FIG. 14, it is a common practice that ultrahigh-precision flat-surface machining disclosed in Japanese Patent Laid-Open No. 1996-257782 is applied to the respective contact surfaces between the mirror cooling plates 55 and 56 and the corresponding mirrors so that profile deformations of the mirrors 55 and 56 are prevented.
FIG. 17 is an exploded perspective view illustrating a lens-holding structure in an optical device, having an exposure apparatus, disclosed in Japanese Patent Laid-Open No. 2002-141270. The foregoing lens-holding structure is configured in such a way that, by flanking a lens 97 with holding members (pressing rings) 91 and 92 each having three protrusions 95 that face the respective corresponding protrusions, the holding members (pressing rings) 91 and 92 are prevented from profile-deforming the lens 97. The foregoing lens-holding structure is configured in such a way that the deformations of holding members 91 and 92 do not cause the bending moment to be applied to the lens (optical element). In addition, reference numerals 96, and 93 and 94 denote a lens tube, and curved surfaces of the lens.
In the resonator illustrated in FIG. 14, the flatness of the contact surface between the mirror presser 53 and the mirror cooling plate 55 and the flatness of the contact surface between the mirror presser 54 and the mirror cooling plate 56 are low; therefore, it has been a problem that, when the mirror pressers 53 and 54 are pressed against the mirror cooling plates 55 and 56, respectively, and fixed fastened with bolts 64, the mirror pressers 53 and 54 profile-deform the mirror cooling plates 55 and 56, respectively, whereby the mirror cooling plates 55 and 56 profile-deform the mirrors 51 and 52, respectively. Because the flatness of the contact surface between the mirror cooling plate 55 and the case 61 and the flatness of the contact surface between the mirror cooling plate 56 and the case 61 are also low, a phenomenon similar to that described above takes place; thus, when the mirror cooling plates 55 and 56 are fixed to the case 61, the case 61 profile-deforms the mirror cooling plates 55 and 56, whereby the mirrors 51 and 52 are deformed.
In order to prevent the foregoing deformation, by applying ultrahigh-precision flat-surface machining to the sides, facing the case 61, of the mirror pressers 53 and 54 and the mirror cooling plates 55 and 56, the sides, facing the mirror pressers 53 and 54, of the mirror cooling plates 55 and 56, respectively, and the contact surface of the case 61, the profile-deformation can be prevented; however, there exists a problem that the ultrahigh-precision flat-surface machining is extremely expensive, as well as a problem that, due to the restriction on the machining apparatus, a large component such as the case 61 cannot be machined.
With regard to the mirror mounting structure illustrated in FIG. 16, it is required to apply ultrahigh-precision machining (with flatness of not more than 1 μm) to the bend block 83; however, there exists a problem that, due to the restriction on the ultrahigh-precision machining apparatus, a large component such as the bend block 83 cannot be machined. When the ultrahigh-precision machining cannot be applied to the bend block 83, the bend block 83 profile-deforms the mirror 82, whereby the flatness does not meet a tolerance.
In the optical-device holding structure illustrated in FIG. 17, because the lens 97 and each of the holding members 91 and 92 are in point contact with each other, the heat-transfer area is extremely small; therefore, the coolability of the lens 97 becomes insufficient. Additionally, in the case where the optical element is formed of a relatively soft material such as ZnSe, because the stress that is concentrated in the vicinity of the protrusion 95 recesses the optical element or partially changes the refraction index thereof, it is required that the optically effective region (e.g., the region through which a light beam passes) and the three point-contact protrusions be separated from each other; therefore, as a result, the diameter of the optical element becomes large, whereby the cost is considerably raised.
Moreover, in the case where an O-shaped ring is provided between the optical element and the holding member in order to keep the airtightness, there exists a problem that, because the optical element and each of the holding members are in point contact with each other, the reactive force, which is produced when the O-shaped rings are crushed, warps the portion, of the optical element, which is not in point contact with the holding members. It is devised that, in order to reduce the warp, the thickness of the optical element is enlarged so that the bending rigidity of the optical element is raised; however, because the optical element becomes extremely expensive, the cost is considerably hiked.